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Topic: Small loop antennas @ 433 Mhz (Read 24018 times)

A purely resistive 50 ohm dummy load would have superior impedance matching to the radio over any of the commercial or home brew antennas for these radios.

But what's the point? Sure 100% of tx power gets transmitted to the antenna and ... burned off as heat. No matter how bad your match with a real antenna - you'll always come out better than with a resistor. 100% of 0 is still 0.

But yeah they have their place - as long as its not sending or receiving rf

A purely resistive 50 ohm dummy load would have superior impedance matching to the radio over any of the commercial or home brew antennas for these radios. If I'm bench-testing between two or more radios the resistor is also a far more compact solution and more than adequate to test comms in close quarters.

With an in-line rf power meter, dummy loads are also the best option to verify the transmitter's output power.

They have their place, but they finish last when you're after an antenna system to achieve long-range comms.

The resistor will have a finite size, also some inductance and capaitance. It'll act like a very, very low efficiency antenna with a very, very high loss as almost all of the power is converted into heat. The radios are very sensitive nowadays, so it will pick up and transmit some signals but its gain will be through the floor. You might on the bench be able to simulate receiving from a long distant transmitter,or it being a transmitter that is a long way away from its receiver when the devices are right next to each other, but for any other practical purpose they are useless as antennae.Mark.

While I'm waiting for the new round board with drills for the air trimmers and two independent loops I built a fully assembled version of the first two boards but with the trimmer instead of fixed caps:

This works really well:

I wrote a little utility which sends packets at different frequencies across the spectrum and use the gateway to capture RSSI. That way I can adjust the trimmer without even attaching the VNA and with a fully live setup.

Both boards work well, close to the performance of Moteinos, Tinos etc. The rectangular model has smaller bandwidth than the round design. It might be the vias in the round one that degrade its Q and thus its performance. That had been a problem also when I measured it with the VNA. But it's still as good overall because the larger loop means more radiation resistance.

What's best is - and this is what I wanted to test with these boards - these loops are really incredibly stable to environmental changes. I've had them on wood, on the floor, on the balcony, in hand etc with very, very minor shift in resonant frequency.

For me this proves that these high Q loops are viable without active tuning if you use air trimmers.

Not really - these digitally adjustable ones have fairly low Q's and are fairly unstable. So efficiency would be worse and you'd have to have some auto-tuning mechanism to keep the resonance right in the face of temp changes.

Insead of a switched capacitor ladder, what if it was just a capacitor ladder where you cut traces (or perhaps burned out fuses) to zero in on the correct capacitance? Would that also have inadequate Q?

@WhiteHare Those digital switch capacitors unfortunately don't work well at UHF, the one you mentioned has large increase in capacitance even at 500MHz. The internals appear to become capacitive at higher frequencies.Mark.